Probably the easiest
rotatable beam is a 2 Element Yagi, both in the mechanical and the
electrical sense. While the gain won't be "up there" with the big
tri-banders & the like, a useful amount of gain can be realized
without breaking the bank. Below is a "sketch" of a 2 Element
Yagi.

Gain is about
5dBd.

The Reflector and the
Driven Element are slightly different lengths, in actuality, the Reflector
is 5% longer than the Driven Element.

The formulas for the element
lengths are listed below:

DE (ft) = 470/F(MHz)
[HF] : DE (in) = 5600/F(MHZ) [VHF]

REF (ft) =
494/F(MHz) [HF] : REF (in) = 5880/F(MHz)
[VHF]

The Spacing can vary from 0.15-Wave to 0.25-Wave,
with little change in the array gain. According to the charts in the
"ARRL Antenna Book", 14th Edition, the variation in gain is less than
0.5dB. What does change is the feed
impedance.

Depending on
how you want to feed the array will determine the spacing. The
closer the spacing, the lower the feedpoint impedance. At
0.20-Wave, the impedance is around 40-Ohms, resulting in a 1.25:1 SWR, and
at 0.25-Wave, the feedpoint impedance is around 60-Ohms, resulting in a
1.17:1 SWR.This is with
direct feed with 50-Ohm cable.The feed can be direct, or through a 1:1 balun, and will show
very little variation either way.

(Editor's note....by extrapolation,
you should get very near 50 ohm impedance if the spacing is about .225
wavelength between reflector and driven element. Swr should be near
1:1)

Construction of the 2 Element Yagi

Various construction methods can be
used for a 2 Element Yagi. The most common is to take a length of
rigid tubing, cut to the desired boomlength, and mount the elements with
U-bolts, pipe clamps, muffler clamps or what-have-you. The boom can
also be wood or a lattice-like structure, or even PVC pipe. The main
consideration is mechanical strength. The elemenst can be any
conductive material that will support its own weight, or a suitable length
of wire or braid that is supported by a rigid non-conductor. One of
the really inexpensive antennas I saw in a publication used lengths a #14
house wire, taped to bamboo poles. Of course, the author lived in
the South, and just went out in his backyard and cut his own canes!
Some people have all the
luck!

Below, I have dimensions for some of the HF and
VHF bands, and some construction "tips":

Let's say, for example, that you need a 10 Meter beam.
Looking at the chart above, (numbers inred), the
longest element is just under 18', and the boomlength would be about 7' to
8.66'. The elements could be DIY aluminum tubing (expensive), or it
could be EMT (conduit), which is heavy, but inexpensive.The boom could
be a length of TV mast, Chain-link fence rail or a 2 x 4.The least
expensive would be the heaviest (2 x 4 boom & EMT elements), but the
total cost would be $LESS.

With about 5dBd gain, and a 25 watt rig (Uniden or Radio
Shack, etc), this would be like going with a 75 watt amp, at $0.33/watt,
and if you were running 100 watts, you would have effectivly 300 watts for
$0.08/watt, plus the cost of the rig, of course.Any way you look at
it, it's a big bang for the Buck! (NOTE, prices are for comparision
only)

If you wanted to make the Yagi for, say, 20 Meters, the
"boom"could be a ladder, (No, I'm not kidding!), or a lattice-construct
made with 2 x 2's or 2 x 4's. It would be HEAVY, ... but the price
of 20 Meter beams are HEAVY, also. It might be feasable to do the
ladder boom, and beef up the mast instead of depleteing your
budget.

For the VHF
beams, the boom could be PVC pipe, 2 x 2's, TV mast or whatever. One
suggestion that was in "73" Magazine awhile back was to use threaded
elements, drill holes in the PVC pipe and "bolt" the elements through the
pipe wall. This would provide the mechanical support and the
insulation for the feedpoint all at the same time. For vertical
polarization, the boom could be extended back beyond the Reflector, and
the beam then end-mounted, putting the mast out of the field of the
antenna. This would allow the antenna to even be side-mounted on an
exsisting mast.

The beamwidth
of a 2 Element Yagi is about 110-Degrees, so aiming is not critical,
however, the front-to-side ratio and front-to-back ratio is 10 - 20 dB,
providing a high degree of rejection to unwanted signals.To put this
in perspective, say you lived in Brockport, NY or somewhere in
the Western edge of Monroe County. The 2 Meter version of this
antenna could be mounted up in the clear, and "sited" on the center of
Rochester. You would have gain from the North Greece area, all the
way around to Rush, and would reduce the VE3 repeaters by a couple-or-3
"S"-units. Not bad for a few chunks of wire & a piece of
PVC.

The possibilities are
limitless. Consider a 2 Element Yagi this Antenna
season.

73, Keith
WB2VUO

Editor's note:

Many years ago, the 10 meter version of this 2
element Yagi was built by myself using much the same construction
techniques as mentioned in the article using small aluminum tubing. It was
installed on a short mast about 15 to 17 feet high with a rotor on
the ground. This turned out to be a fantactic performing antenna and I
made contacts worldwide on it. I still have the "parts and pieces" of it
resting comfortably in my "junk" pile as it is called by the
XYL!I hope to revive it from it's "vacation" someday! Thanks to Keith,
WB2VUO for sharing his article with us! Contact Keith via his QRZ email
address.....N4UJW